Solid electrolyte for use in oxygen concentration sensor

ABSTRACT

A solid electrolyte for an oxygen concentration sensor is provided by mixing ZrO 2  and about 6.5 to 7% by mol of a stabilizer such as Y 2  O 3 , grinding the mixture to give a BET specific surface area of about 9 to 20 m 2  /g, adding to the ground mixture a forming aid, forming the mixture into a predetermined configuration and firing the formed mixture at about 1420° to 1480° C. The electrolyte has a surface-average grain size of about 5 microns or less and a structure of a cubic phase and a monoclinic and/or tetragonal phase at the surface thereof and, furthermore, may contain about 5% or less of a monoclinic phase per the total of the cubic and monoclinic phases, in a finely ground state.

BACKGROUND OF THE INVENTION

This invention relates to an oxygen sensor used in detecting the oxygen concentration in a gas such as the exhaust gas from an internal combustion engine of automotive vehicles. More particularly, it relates to a solid electrolyte consisting essentially of sintered zirconia, which has a good stability of crystal structure and a good oxygen ion conductivity.

Among the solid electrolytes of this sort for use in an oxygen sensor, there has been known a sintered body of partially stabilized zirconia which is prepared by mixing zirconia with a stabilizer such as yttria, calcia, or ytterbia followed by sintering at high temperatures and has a mixed structure of a monoclinic and/or tetragonal phase and a cubic phase, as disclosed in U.S. Pat. No. 4,219,359 (Miwa et al.).

This known sintered body of partially stabilized zirconia has a high strength and excellent thermal shock resistance, because it has a fine structure and is a composite material.

However, it has been found that the composite material causes the structure of the partially stabilized zirconia to be unstable. That is, the proportions of the tetragonal phase content and the monoclinic phase content in said sintered body are variable at the temperatures of using a solid electrolyte for an oxygen sensor, i.e., about -40° to 900° C. The repeated changes in temperature within the range of about -40° to 900° C., or the continuous use of a sensor at a lower temperature range, e.g., about 200° to 300° C. cause the phase transition from the tetragonal phase to the monoclinic phase to take place. Furthermore, a stable cubic phase is partially transformed to the monoclinic phase.

Since the phase transition to the monoclinic phase is accompanied by volume expansion, it causes the structure of the sintered body to be breakable. In addition, since the monoclinic phase content is increased by the phase transition, the sintered body becomes poor in an oxygen ion conductivity. Thus, the service temperature of an oxygen sensor is biased to a higher, narrowly restricted range.

Therefore, it is an important problem how to reduce the monoclinic phase content in a solid electrolyte made of the partially stabilized zirconia, i.e., to prevent the monoclinic phase content from being increased by the phase transition during service.

Thus, the inventors have found that when the partially stabilized sintered zirconia was finely ground, there may be detected a monoclinic phase.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide a solid electrolyte for use in an oxygen sensor exhibiting an excellent thermal shock resistance as well as a good oxygen ion conductivity.

Other objects and features of this invention will become apparent from the following descriptions.

DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

The solid electrolyte of this invention is provided by maintaining the monoclinic phase content of the sintered zirconia in a pulverized state at up to about 5% by weight based on the total of both the phases (the remainder being a cubic phase) and the surface-average grain size before grinding, at about 5 microns or less, thereby stabilizing the structure and preventing the sintered body from destruction of the structure. In order to maintain the surface grain size at or below about 5 microns and the proportion of monoclinic phase in the sintered material at or below about 5% by weight, as specified in this invention, the concentration of a stabilizer such as yttria and the sintering temperature should be properly selected; namely, the concentration of a stabilizer such as yttria is selected within the range of from about 6.5 to 7% by mol (the remainder being zirconia), preferably from about 7 to 9% and the sintering temperature of the material within the range of from about 1420° to 1480° C.

The technical-grade zirconia being used as a raw material in manufacturing the solid electrolyte of this invention contains, as a rule, certain amounts of impurities such as hafnium oxide, titanium oxide, iron oxide, and so on. Therefore, the constituents of the solid electrolyte of this invention are permitted to contain the above-noted impurities.

Furthermore, in preparing the sintered zirconia according to this invention, a sintering promoter such as aluminum oxide and/or silicon oxide can be used. The suitable amount added of a promoter ranges from about 1 to 5% by weight for aluminum oxide and from about 0.2 to 2% by weight for silicon oxide, said amounts being based on the total weight of zirconia and the stabilizer such as yttria.

An example of the structure of an oxygen sensor employing the solid electrolyte of this invention is described below by referring to the accompanying drawings.

FIG. 1 is a schematic drawing to illustrate the method employed in this invention for measuring the surface-average particle size.

FIG. 2 is an electric connecting diagram to illustrate the method employed in this invention for measuring the resistance.

FIGS. 3 and 4 are microphotographs of a solid electrolyte to illustrate the invention.

FIG. 5 is a partial sectional view illustrating one example of the construction of an oxygen sensor employing the solid electrolyte of this invention.

In FIG. 5, 1 is an oxygen concentration detecting element formed from the solid electrolyte comprising about 93% by mol of zirconia and about 7% by mol of yttria in the form of solid solution. The element 1, in a cup-like form open at one end and closed at the other, is coated on inner and outer surfaces with porous layers of the first electrode 2 and the second electrode 3, which are composed of platinum and are applied by chemical plating, vacuum deposition or paste baking. The second electrode is covered with a heat-resistant porous protective coating of metal oxides such as magnesia-alumina spinel (MgAl₂ O₄) and the like, in the area where it is exposed to the exhaust gas. Tubular housing 5 made of a heat-resistant metal is provided around the outer surface of the oxygen concentration detecting element 1. In the annular space between the housing 5 and the oxygen concentration detecting element 1, there are disposed conductive graphite powder 6, asbestos ring 7, and conductive metallic ring 8. One end of protective tube 9 made of a conductive heat-resistant metal, e.g. stainless steel, is inserted into said annular space. This end of the protective tube 9 carries spacer ring 10 of a conductive metal supported on flange 9a. The upper edge of housing 6 is fixed to the protective tube 5 by calking to hold in place both the housing 5 and the protective tube 9 against the oxygen concentration detecting element 1. The housing 5 is provided with flange 11 to fix the device to the exhaust line (not shown). The area of the outer surface of oxygen concentration detecting element 1, where it is not exposed to the exhaust gas, is covered with a portion of the protective tube 9, as mentioned above. Stem 12, made of a conductive metal, e.g. stainless steel, and having through-hole 12a along the center line, is fixed, through conductive graphite powder 13, to the inner wall of the oxygen concentration detecting element 1. Insulator 14 of alumina or the like is disposed in the upper end portion of the protective tube 9 and held in place by calking the upper edge of the protective tube. Hollow tube 15 made of a conductive metal, e.g. stainless steel, is inserted through the insulator 14 and spring 16 is mounted between the flange 15a of tube 15 and the shoulder 12a formed in the stem 12. The stem 12 is tightly fixed in place by being pressed against the oxygen concentration detecting element 1 by the pressure exerted from the set load of the spring 16. A piece of stainless steel wire 17 is fixed by welding at one end thereof to the wall of through-hole 12a of the stem 12 and welding at the other end to the upper insider wall of hollow tube 15. A leading wire 19 from connector 18 is connected to the end of the hollow tube 15 opposite to the flange 15a by calking the terminal piece 20. A duplex protective tube 21 having a large number of small holes 21a is fixed to the lower end of housing 5 so as to cover the area of outer surface of the oxygen concentration detecting element 1, where it is exposed to the exhaust gas. The first electrode 2 of the oxygen concentration detecting element 1 is electrically connected to the leading wire 19 through the graphite powder 13, stem 12, stainless steel wire 17, and hollow tube 15 or intervening spring 17. The second electrode 3 is electrically connected to the housing 5 through the conductive graphite powder 6 and conductive ring 8. Heat-resistant rubber tubing 22 is fitted around the protective tube 9 through the intervening collar 23.

The invention is illustrated below in detail with reference to Example in which yttrium oxide was used as the stabilizer, though other metallic oxides such as, for example, calcium oxide, magnesium oxide, scandium oxide and ytterbium oxide may be used.

EXAMPLE

Commercially available zirconium oxide of about 99.5% in purity, containing a small amount of hafnium oxide (HfO₂) and having a BET specific surface area of about 2.5 m² /g was blended with commercially available yttrium oxide of about 99.9% in purity, having a BET specific surface area of about 3.8 m² /g, in predetermined molar ratios as shown in Table 1. The resulting mixture was ground so as to obtain a powder having a specific surface area of about 12 m² /g. The powdered mixture was mixed with a forming aid such as polyvinyl alcohol or the like to form a slurry containing about 40 to 45% by weight of water. The slurry was spray-dried and molded into a rectangular, disc or tubular specimen by means of an oil press at a pressure of about 1 ton/cm². Each specimen was fired at a predetermined temperature, as shown in Table 1, for about 3 hours under an oxidizing atmosphere at normal pressure. The fired rectangular specimen was tested for the flexural strength, stability of structure, and surface-average grain size by the methods described later. The specimen in a disc form was polished to obtain a mirror-finished surface and the monoclinic phase content of the surface was determined; the monoclinic phase content of the pulverized specimen was also determined. Another disc specimen was ground to a thickness of 2.5 microns, then coated on both sides with a silver paste in a circular area of 200 mm in diameter, and baked. The disc thus treated was tested for the resistance at a temperature of about 400° C. and a content current of 10 μA (measurement was made after the lapse of about 10 minutes from the time when the temperature reached about 400° C. and the electric current was turned on). The tubular specimen was evaluated for the thermal shock resistance according to the methods and test conditions as described below.

The procedures and conditions of each test were as described below.

(a) Flexural strength: A sintered body was cut and ground to a rectangular rod of 5×8×60 mm in size and tested for the flexural strength by the known threepoint bending test method.

(b) Stability of structure: After heating a rectangular sintered body at 200°±15° C. for 1000 hours, the fracture was examined by a scanning electron microscope (SEM) at 10,000 magnifications to determine the presence or absence of intergranular rupture caused by volume expansion originated from the phase transition.

(c) Monoclinic component content: A sintered body was finely pulverized to a particle size below 10 microns, whereby, if a tetragonal phase was present in the sintered body, the tetragonal phase was completely transformed into a monoclinic phase and the monoclinic component content was calculated by the equation:

    {I[M(111)+M(111)]/I[M(111)+M(111)+C(111)]}×100=Weight % monoclinic.

When a tetragonal phase is detected at the surface or lapped surface of the sintered body, there arises a question whether or not the tetragonal phase is completely eliminated on pulverizing. With this respect, it was confirmed that after pulverization no X-ray diffraction peak of the tetragonal phase was found, under the following conditions, in the sample containing initially a tetragonal phase, indicating that the phase transition had been completed. In the above equation M and C represent monoclinic and cubic phases, respectively. The X-ray diffraction intensity I is a value integrated by peak-height x half-width.

Conditions for X-ray diffraction:

Target: (Rotary type) Cu

Tube voltage: 40 kV

Tube current: 80 mA

Divergency slit: 1°

Receiving slit: 1 mm

Goniometer speed: 2θ-1°/minute

Chart speed: 20 mm/minute

Time constant: 1 second

Full scale: 4,000 cps-400 cps

Provided with graphite monochrometer

Limit of detection for monoclinic phase: >0.5 weight %

(d) Surface-average grain size: The surface-average grain size was determined on the SEM photograph of the sintered body surface taken at 1,000-10,000 magnifications. A straight line was drawn at random across the photograph, as shown schematically in FIG. 1. The diameter of each of the grains lying across the line was determined in terms of the maximum width (d in FIG. 1) of the grain measured in the direction vertical to said straight line. Another straight line was drawn parallel to the above line with a sufficient space therebetween so as not to cross over the grains which had already been measured and the diameter determination was repeated on all the grains lying across the straight line. The determination was further repeated until the number of measured grains had amounted to 20-50. The average value of the determined diameters was taken as a surface-average grain size.

(e) Resistance: The disc specimen 1 provided with silver electrodes 2 and 3, as shown above, was connected to a circuit shown in FIG. 2 and the resistance R was determined under a constant current load. In FIG. 2, A is an ammeter, V a voltmeter, R a rheostat, and E an electric source.

(f) Thermal shock resistance: The aforesaid tubular specimen of the sintered body was a tube closed at one end having an inner diameter of 4 mm and an outer diameter of 8 mm, which was prepared by molding a granulated material by means of a common hydrostatic press at a pressure of 1 ton/cm², followed by baking. Inside the closed end of the tubular sintered body, was placed the heat sensing junction of a CA thermocouple which was connected to a recorder to record the temperature with the lapse of time, whereby the rate of increase or decrease of temperature (°C./second) could be calculated. The relative position of a burner to the zirconia sintered body was adjusted so that the flame of the burner supplied with a mixed fuel gas of compressed air and a city gas could touch the zirconia sintered body at the closed end and over a length of about 1.5 mm measured from the closed end. The recorder operated simultaneously with the lighting of the fuel gas to determine the rate of temperature change. After this thermal shock test, the cooled zirconia sintered body was dyed with fuchsine to examine the cracking and the rate of temperature increase which caused the cracking was determined for each of the zirconia sintered bodies prepared under varied conditions. The higher the rate of temperature change which caused cracking, the higher the thermal shock resistance was rated.

The results obtained in the example were as shown in Table 1.

                  TABLE 1                                                          ______________________________________                                          Results of evaluation obtained in Example.                                    ______________________________________                                                                Bulk speci-     Stability of                                 Y.sub.2 O.sub.3                                                                         Sinter-  fic gravity     structure                                    concen-  ing tem- after sin-                                                                             Flexural                                                                               (intergran-                             Test tration  perature tering  strength                                                                               ular                                    No.  (mole-%) (°C.)                                                                            (in Hg) (kg/mm.sup.2)                                                                          rupture                                 ______________________________________                                         1    5        1520     5.69    28.6    Yes                                     2             1720     5.72    37.5    ↑                                 3    6        1520     5.65    25.9    ↑                                 4             1720     5.67    27.5    ↑                                 5      6.5    1420     5.62    25.4    No                                      6             1720     5.65    24.2    ↑                                 7    7        1420     5.64    22.8    ↑                                 8             1720     5.68    18.5    ↑                                 9    8        1520     5.70    17.2    ↑                                 10            1720     5.72    14.5    ↑                                 11   9        1520     5.63    15.4    ↑                                 12            1720     5.64    13.9    ↑                                 ______________________________________                                                       Surface-             Thermal                                     Monoclinic content                                                                           average grain                                                                              Resis-   shock re-                                   Surface Powder    size        tance  sistance                                  (wt.-%) (wt.-%)   (microns)   (kΩ)                                                                            (°C./sec.)                         ______________________________________                                         17      21        0.9         7.9    >60                                       3.5     8.2       5.4         7.2    >45                                       4.8     7.4       1.2         5.9    >50                                       Not     5.5       9.5         6.4    >40                                       detected                                                                       1.9     3.2       0.32        3.8    >40                                       Not     1.6       17.5        4.8    >30                                       detected                                                                       0.6     1.8       1.4         1.6    >40                                       Not     Not       25.4        1.5    >25                                       detected                                                                               detected                                                               ↑ ↑   6.7         3.5    >30                                       ↑ ↑   35.4        4.2    >20                                       ↑ ↑   6.9         16     >25                                       ↑ ↑   35.9        19     >20                                       ______________________________________                                    

As is seen from Table 1, there was observed a monoclinic phase in the sintered bodies of test Nos. 1, 2, 3, 5 and 7 at the surfaces thereof and when pulverized. The bodies of test Nos. 1 to 3 were found to be accompanied with the intergranular rupture and to have a high electric resistance. These bodies had a great amount of a monoclinic phase in a pulverized state and a large surface-average particle size.

On the other hand, there was observed no intergranular rupture in the bodies of test Nos. 5 and 7. These bodies had a low electric resistance. These bodies in a pulverized state had a smaller amount of a monoclinic phase, and they had a smaller surface-average particle size.

The SEM photographs of the surfaces of test Nos. 2 and 5 are shown in FIGS. 3 and 4, respectively. The SEM magnifications are 2,000 and hence a length of 20 mm in an actual scale on the photographs corresponds to 10 microns.

In the bodies of test Nos. 4 and 6 there was observed a monoclinic phase only in a pulverized state. Test No. 4 contained a relatively greater amount, i.e. 5.5% of a monoclinic phase and had a relatively larger surface-average particle size of 9.5 microns. As a result, there was observed intergranular rupture in the body of test No. 4, and this body had a high electric resistance. Test No. 6 contained a relatively smaller amount, i.e., less than 5% of a monoclinic phase, but had a relatively larger surface-average particle size of 17.5 microns. The body of test No. 6 was poor in thermal shock resistance.

In the bodies of test Nos. 8 to 12 there were observed no monoclinic phase. These bodies had a considerably lower flexural strength and thermal shock resistance.

On the basis of the results shown in Table 1, the present inventors examined conditions necessary for the sintered body of zirconia to exhibit such desirable characteristics as a low electric resistance and excellent thermal shock resistance without any rupture in structure. As a result, it has been found that the monoclinic phase content of the sintered body in a pulverized state should be about 5% or less and the surface-average grain size of the sintered body prior to the pulverization should be about 5 microns or less in order to obtain such characteristics.

As fully described above, according to this invention a solid electrolyte can be effectively stabilized in structure and kept from failure of the structure; an excellent thermal shock resistance and a high oxygen ion conductivity are also resulted. Thus, the present invention may provide an oxygen sensing solid electrolyte suitable for practical use. 

What is claimed is:
 1. A solid electrolyte for an oxygen concentration sensor, consisting essentially of a sintered body of partially stabilized zirconia having a crystal structure including crystal grains of a cubic phase and a monoclinic and/or tetragonal phase and having a surface-average grain size of about 5 microns or less, said structure being changed, on finely grinding, into a structure in which a cubic phase and a monoclinic phase coexist and the proportion of the monoclinic phase is about 5% or less by weight based on the total of both the phases.
 2. The solid electrolyte according to claim 1, wherein the sintered body of partially stabilized zirconia contains Y₂ O₃ as a stabilizer.
 3. The solid electrolyte according to claim 1, wherein the sintered body of partially stabilized zirconia contains CaO as a stabilizer.
 4. The solid electrolyte according to claim 1, wherein the sintered body of partially stabilized zirconia contains MgO as a stabilizer.
 5. The solid electrolyte according to claim 1, wherein the sintered body of partially stabilized zirconia contains Yb₂ O₃ as a stabilizer.
 6. The solid electrolyte according to claim 1, wherein the sintered body of partially stabilized zirconia contains Sc₂ O₃ as a stabilizer.
 7. The electrolyte according to claim 2, 3, 4, 5 or 6, wherein the stabilizer is contained in an amount of about 6.5 to 7% by mol.
 8. The electrolyte according to claim 2, 3, 4, 5 or 6, wherein the sintered body contains as a sintering promotor about 1 to 5% by weight of Al₂ O₃ or about 0.2 to 2% by weight of SiO₂, both the percentages being based on the total of ZrO₂ and the stabilizer.
 9. A solid electrolyte for an oxygen concentration sensor, consisting essentially of a sintered body of partially stabilized zirconia having a crystal structure including crystal grains of a cubic phase and a monoclinic and/or tetragonal phase and from about 6.5 to about 7 mol percent of a zirconia stabilizer, said zirconia having a surface-average grain size of about 5 microns or less, said structure being changed, on finely grinding, into a structure in which a cubic phase and a monoclinic phase coexist and the proportion of the monoclinic phase is about 5% or less by weight based on the total of both the phases. 